ABSTRACT Human oral plaque is a polymicrobial community whose composition varies during health and disease. Recent advances in technology have yielded new information on the identity and abundance of constituent species during these conditions yet do not allow predictions of direct interactions between specific organisms. The biogeography of reproducible structures within supragingival plaque has been characterized through advanced microscopy methods suggesting key organisms that may help arrange plaque structure. Species-species co-proximity within these ordered structures is suggestive of metabolite mediated interactions between them. Our global hypothesis is that adjacent species in healthy plaque biofilms have specific metabolic and physical interactions that shape both the physical and compositional structure of this community. To identify mechanistic interactions between adjacent species, we must determine which species exist together in vivo. Preliminary data indicates that the highly abundant Corynebacterium matruchotii and Haemophilus parainfluenzae bacterial species exist directly adjacent to several Streptococcus spp. suggesting they must be able to tolerate pH and oxidative stress produced by oral streptococci. Direct imaging further suggests that some species bind directly to each other or mutually to a host intermediate such as salivary protein. The physical means of attachment for C. matruchotii and H. parainfluenzae to Streptococcus spp. are unknown. This proposal will determine which Streptococcus species interact with C. matruchotii and H. parainfluenzae via microscopy (Aim 1). It will also determine if C. matruchotii and H. parainfluenzae participate in cross- feeding interactions with streptococcal produced metabolites and will identify mechanisms that either bacterium uses to tolerate pH and oxidative stress through transcriptome analyses, mutant library assays, and quantitative metabolite measurements (Aim 2). Lastly, our proposal will determine physical factors produced by bacteria that are responsible for co-adhesion between different species. These will be identified by direct and random mutagenesis of C. matruchotii and H. parainfluenzae and tested in combination with known interacting Streptococcus species. The goal of this proposal is to identify interacting species in healthy supragingival plaque and characterize mechanistic interactions between them, revealing how they may contribute to plaque structure and composition. Our rationale is that we will identify mechanisms that promote interactions between highly abundant organisms in healthy plaque and identify candidate species and their associated interactions for use in probiotic or prophylactic interventions to manage oral plaque communities to prevent opportunistic infection.